This invention relates to a distillate material having a high cetane number and useful as a diesel fuel or as a blending stock therefor, as well as the process for preparing the distillate. More particularly, this invention relates to a process for preparing distillate from a Fischer-Tropsch wax.
Clean distillates that contain no or nil sulfur, nitrogen, or aromatics, are, or will likely be in great demand as diesel fuel or in blending diesel fuel. Clean distillates having relatively high cetane number are particularly valuable. Typical petroleum derived distillates are not clean, in that they typically contain significant amounts of sulfur, nitrogen, and aromatics, and they have relatively low octane numbers. Clean distillates can be produced from petroleum based distillates through severe hydrotreating at great expense. Such severe hydrotreating imparts relatively little improvement in cetane number and also adversely impacts the fuels lubricity. Fuel lubricity, required for the efficient operation of fuel delivery system, can be improved by the use of costly additive packages. The production of clean, high cetane number distillates from Fischer-Tropsch waxes has been discussed in the open literature, but the processes disclosed for preparing such distillates also leave the distillate lacking in one or more important properties, e.g., lubricity. The Fischer-Tropsch distillates disclosed, therefore, require blending with other less desirable stocks or the use of costly additives. These earlier schemes disclose hydrotreating the total Fischer-Tropsch product, including the entire 700xc2x0 F.xe2x88x92fraction. This hydro-treating results in the elimination of oxygenates from the distillate.
By virtue of this present invention small amounts of oxygenates are retained, the resulting product having both very high cetane number and high lubricity. This product is useful as a diesel fuel as such, or as a blending stock for preparing diesel fuels from other lower grade material.
In accordance with this invention, a clean distillate useful as a diesel fuel or as a diesel fuel blend stock and having a cetane number of at least about 60, preferably at least about 70, more preferably at least about 74, is produced, preferably from a Fischer-Tropsch wax and preferably derived from a cobalt or ruthenium catalyst, by separating the waxy product into a heavier fraction and a lighter fraction; the nominal separation being at about 700xc2x0 F. Thus, the heavier fraction contains primarily 700xc2x0 F.+, and the lighter fraction contains primarily 700xc2x0 F.xe2x88x92.
The distillate is produced by fisher separating the 700xc2x0 F.xe2x88x92 fraction into at least two other fractions: (i) one of which contains primary C12+ alcohols and (ii) one of which does not contain such alcohols. The fraction (ii) is preferably a 500xc2x0 F.xe2x88x92 fraction, more preferably a 600xc2x0 F.xe2x88x92 fraction, and still more preferably a C5-500xc2x0 F. fraction, or a C5-600xc2x0 F. fraction. This fraction (ii) and the heavier fraction are subjected to hydroisomerion in the presence of a hydroisomerization catalyst and at hydroisomerization conditions. The hydroisomerization of these fractions may occur separately or in the same reaction zone, preferably in the same zone. In any event at least a portion of the 700xc2x0 F.+ material is converted to 700xc2x0 F.xe2x88x92 material. Subsequently, at least a portion and preferably all of the 700xc2x0 F.xe2x88x92 material from hydroisomerization is combined with at least a portion and preferably all of the fraction (ii) which is preferably a 500-700xc2x0 F. fraction, and more preferably a 600-700xc2x0 F. fraction, and is further preferably characterized by the absence of any hydrotreating, e.g, hydroisomerization. From the combined product a diesel fuel or diesel blending stock boiling in the range 250-700xc2x0 F. is recovered and has the properties described below.